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THE BEGINNING OF HUMAN DEVELOPMENT THE FIRST WEEK

THE BEGINNING OF HUMAN DEVELOPMENT THE FIRST WEEK. Prof.Dr .Meltem ÖZGÜNER Department of Histology - Embryology. CLEAVAGE OF THE ZYGOTE. Cleavage , consists of repeated mitotic divisions of the zygote . Cleavage , results in a rapid increase in the number of cells .

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THE BEGINNING OF HUMAN DEVELOPMENT THE FIRST WEEK

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  1. THE BEGINNING OF HUMAN DEVELOPMENTTHE FIRST WEEK Prof.Dr.Meltem ÖZGÜNER Department of Histology -Embryology

  2. CLEAVAGE OF THE ZYGOTE • Cleavage,consists of repeatedmitoticdivisions of thezygote. • Cleavage, results in a rapidincrease in thenumber of cells.

  3. CLEAVAGE OF THE ZYGOTE

  4. CLEAVAGE OF THE ZYGOTE • Any of the cells resulting from the cleavage of a fertilized ovum during early embryonic development is known as blastomeres

  5. CLEAVAGE OF THE ZYGOTE • Cleavage,normallyoccurs as thezygotepassesalongtheuterinetubetowardtheuterus.

  6. CLEAVAGE OF THE ZYGOTE • Thezygote is contained a thick zona pellucidaduringcleavage; sothenumber of thecells (blastomeres) increases but thesizes of thecellsaresmaller.

  7. CLEAVAGE OF THE ZYGOTE • Division of thezygoteintoblastomeresbeginsabout 30 hoursafterfertilization. • Afterthethirddivision (8-cellstage), theblastomereschangetheirshapeandtightlyalignthemselvesagainsteachotherto form a compactball of cellsknown as the morula.

  8. CLEAVAGE OF THE ZYGOTE • COMPACTION:Thephenomenon of tightlypackaging, known as compaction. • Compaction is probablymediatedbycellsurfaceadhesionglycoproteins

  9. CLEAVAGE OF THE ZYGOTE • Compactionpermitsgreatercell-to-cellinteractionand is a prerequisiteforsegregation of thecellsthat form theembryoblastorinnercellmass of blastocyst.

  10. CLEAVAGE OF THE ZYGOTE • MORULA; is a sphericalsolidball of 12 ormoreblastomeres, formsaboutthreedaysafterfertilizationandenterstheuterus.

  11. CLEAVAGE OF THE ZYGOTE • Itwasgiventhe name of morula because of itsresemblencetothefruit of themulberrytree.

  12. FORMATION OF THE BLASTOCYST • Shortlyafterthe morula enterstheuterus (aboutfourdaysafterfertilization) spacesappearbetweenthecentralblastomeres of the morula. • Fluidsoonpassesthroughthe zona pellucidaintothesespacesfromtheuterinecavity

  13. FORMATION OF THE BLASTOCYST As thefluidincreases, it seperatestheblastomeresintotwoparts: • 1- a thinouterlayercalledthetrophoblast(Gr.trophe, nutrition) whichgivesrisetopart of theplacenta • 2- a group of centrallylocatedblastomeres, known as theinnercellmass(orembryoblast) whichgivesrisetotheembryo

  14. FORMATION OF THE BLASTOCYST • 3-Thefluid-filledspaceswithintheblastocystsoonfuseto form a single, largeblastocystcavity • AT THIS STAGE OF DEVELOPMENT, THE CONCEPTUS IS CALLED A BLASTOCYST (BLASTULA)

  15. FORMATION OF THE BLASTOCYST • Embryoblastprojectsintotheblastocystecavityandthetrophoblastformsthewall of blastocyst

  16. FORMATION OF THE BLASTOCYST • Aftertheblastocystfloated in theuterusforabouttwodays, the zona pellucidagraduallydegeneratesanddisappear • Thispermitstheblastocysttoincrease in size

  17. FORMATION OF THE BLASTOCYST • Whilefloatingfreely in theuterus, it derivesitsnourishmentfromthesecretions of theuterineglands.

  18. FORMATION OF THE BLASTOCYST • Aboutsixdaysafterfertilization(about 20-28 day of a menstruelcycle) theblastocystattachestotheendometrialepithelium, usuallyfromtheembryonicpole.

  19. FORMATION OF THE BLASTOCYST • As soon as blastocystattachestotheepithelium, thetrophoblastsproliferaterapidlyanddifferentiates in totwolayers: 1-an innercytotrophoblast 2- an outersyncytiotrophoblast

  20. FORMATION OF THE BLASTOCYST • Thefingerlikeprocesses of thesyncytiotrophoblastextendthroughtheendometrialepitheliumandinvadetheendometrialconnectivetissue (stroma)

  21. THE END OF THE FIRST WEEK • Bytheend of thefirstweek, theblastocyst is superficiallyimplanted in thecompactlayer of endometriumand is derivingitsnourishmentfromtheerodedmaternaltissues • At about seven days, a flattenedlayer of cellscalledthehypoblast (primitive endoderm) appears on thesurface of theinnercellmassfacingtheblastocystcavity

  22. THE END OF THE FIRST WEEK

  23. SUMMARY OF THE FIRST WEEK

  24. MONOZYGOTIC TWINS Figure 3-13 Modes of monozygotic twinning. A, Cleavage of an early embryo, with each half developing as a completely separate embryo. B, Splitting of the inner cell mass of a blastocyst and the formation of two embryos enclosed in a common trophoblast. This is the most common mode of twinning. C, If the inner cell mass does not completely separate, or if portions of the inner cell mass secondarily rejoin, conjoined twins may result.

  25. TOTIPOTENT STEM CELLS

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